The present invention relates to cellular communication systems, and more particularly to support for both full-bandwidth and limited-bandwidth devices in a cellular communication system.
Cellular communication systems typically comprise a land-based network that provides wireless coverage to mobile terminals that can continue to receive service while moving around within the network's coverage area. The term “cellular” derives from the fact that the entire coverage area is divided up into so-called “cells”, each of which is typically served by a particular radio transceiver station (or equivalent) associated with the land-based network. Such transceiver stations are often referred to as “base stations”. As the mobile device moves from one cell to another, the network hands over responsibility for serving the mobile device from the presently-serving cell to the “new” cell. In this way, the user of the mobile device experiences continuity of service without having to reestablish a connection to the network. FIG. 1 illustrates a cellular communication system providing a system coverage area 101 by means of a plurality of cells 103.
The radiofrequency spectrum that is utilized to provide mobile communication services is a limited resource that must be shared in some way among all of the users in a system. Therefore, a number of strategies have been developed to prevent one mobile device's use (both transmitting and receiving) of radio spectrum from interfering with that of another, as well as to prevent one cell's communications from interfering with those of another. Some strategies, such as Frequency Division Multiple Access (FDMA) involve allocating certain frequencies to one user to the exclusion of others. Other strategies, such as Time Division Multiple Access (TDMA) involve allowing multiple users to share one or more frequencies, with each user being granted exclusive use of the frequencies only at certain times that are unique to that user. FDMA and TDMA strategies are not mutually exclusive of one another, and many systems employ both strategies together, one example being the Global System for Mobile communication (GSM).
As designers strive to develop systems with higher and higher capabilities (e.g., higher communication speeds, resistance to interference, higher system capacity, etc.), different technical features are incorporated, including different means for sharing radiofrequency resources. To take one of a number of possible examples, the Evolved-Universal Terrestrial Radio Access Network (E-UTRAN) Long Term Evolution (LTE) technology, as defined by 3GPP TR 36.201, “Evolved Universal Terrestrial Radio Access (E-UTRA); Long Term Evolution (LTE) physical layer; General description” will be able to operate over a very wide span of operating bandwidths and also carrier frequencies. Furthermore, E-UTRAN systems will be capable of operating within a large range of distances, from microcells (i.e., cells served by low power base stations that cover a limited area, such as a shopping center or other building accessible to the public) up to macrocells having a range that extends up to 100 km. In order to handle the different radio conditions that may occur in the different applications, Orthogonal Frequency Division Multiple Access (OFDMA) technology is used in the downlink (i.e., the communications link from the base station to the User Equipment—“UE”) because it is a radio access technology that can adapt very well to different propagation conditions. In OFDMA, the available data stream is portioned out into a number of narrowband subcarriers that are transmitted in parallel. Because each subcarrier is narrowband it only experiences flat-fading. This makes it very easy to demodulate each subcarrier at the receiver.
Cellular systems like Wideband Code Division Multiple Access (WCDMA) and LTE regularly transmit information directed at all communication devices within the cell; this information is referred to as “broadcast information.” Such broadcast information is needed in order for the mobile terminal to be able to determine permission to camp on a cell as well as connection setup parameters. In both LTE and WCDMA the broadcast information is divided into a Master Information Block (MIB) and a System Information Block (SIB). The MIB includes the most important system information, such as frame timing. The MIB could also, as in WCDMA, give a pointer to time instances for when different SIBs are transmitted on the Primary Common Control Physical Channel (P-CCPCH). In LTE however, the SIB redundancy versions are always transmitted in a certain order, on the PDSCH in sub frame 5. In both WCDMA as well as LTE, the frequencies allocated to SIB transmission(s) are, in a sense, defined by standardization, so there is no provision for the MIB to include any such information. Information identifying the timing and frequencies to be used for SIB transmissions can be communicated in control channel information that is broadcast at a known time. The SIB then includes more cell specific information, such as information needed for the mobile terminal to be able to perform random access, via the Random Access Channel (RACH). Such RACH-related information could include, for example, which uplink time slots to use for RACH transmissions as well as RACH power parameters and RACH signatures to be used.
In systems such as WCDMA and LTE, the MIB is always transmitted at a certain time instant, and always occupies a standard-defined frequency allocation. Consequently, each mobile terminal is designed to know how to find the MIB. For instance, in LTE the MIB is transmitted in the central 6 resource blocks (RBs) in subframe 0, and consists of four parts, each of which can be individually recovered (provided that the received signal has a sufficiently good signal-to-noise ration (SNR)). The SIB redundancy versions are, as mentioned above always transmitted in subframe 5, in a specific order. However, the frequencies used could vary over the whole system bandwidth between different frames. The position used for the SIB redundancy versions are determined by decoding the Physical Downlink Control Channel (PDCCH), which points out which RBs of the Physical Downlink Shared Channel (PDSCH) are used for SIB transmission(s). The modulation and coding scheme (MCS) used for the SIBs are also signalled on the PDCCH.
It will be appreciated from the above that, in order to establish a connection with a cell, a standard-compliant communication device needs to be able to receive and decode the PDCCH. The PDCCH itself occupies the whole system bandwidth, which in turn forces current LTE (and similar) terminals to adapt its receiver bandwidth to match that of the system bandwidth in order to be able to receive the SIB, if any such information is transmitted to the UE or broadcasted to multiple UEs in the subframe (1 ms period). This raises issues and problems, as will now be described.
Machine type communication (MTC) over LTE is increasingly gaining attention as operators are planning for replacement of older communication systems, like GSM, by LTE networks. MTC devices, such as connected sensors, alarms, remote control devices and the like, are common in GSM networks where they co-exist with more conventional UEs (e.g., mobile phones). MTC devices are generally characterized by a modest bit rate and sparse communication activity. The number of MTC devices is expected to increase dramatically during the next few years.
In release 8/9 versions of LTE, the supported cell bandwidth is within the range of about 1.4 to 20 MHz (6 and 100 resource blocks (RBs) in LTE terminology). As mentioned above, LTE's PDCCH extends over the full cell bandwidth, which means that all UEs have to support reception over the full cell bandwidth in order to receive control information and thereby be able to establish a connection with the cell.
LTE Release 8 already supports time-domain multiplexing on a subframe basis between unicast and multicast signaling of subframes used for Multimedia Broadcast via Single Frequency Network (MBSFN) in order to allow MBSFN to be introduced in later releases without negatively impacting legacy terminals. Any terminal (UE) designed in accordance with earlier versions of LTE (a “legacy terminal”) does not support MBSFN but does recognize that subframes signaled as being MBSFN subframes contain nothing for the terminal to receive, and hence reception can be avoided in those subframes. One exception is the first OFDM symbol in the subframe which carries cell-specific reference signals (CRS), which may be used by the terminal (e.g. for channel estimation or for measurements (e.g. Radio Link Monitoring—“RLM” or Reference Signal Received Power—“RSRP”), particularly when adjacent to normal subframes. MBSFN subframes are now being discussed for use not only for multicast operation, but in the context of relaying and improved measurements and improved performance in heterogeneous network deployment scenarios along with Almost Blank Subframes (ABS).
MTC devices utilizing a cellular system for communication have become increasingly popular. The notion of developing an MTC device that is capable of communicating by means of communication systems such as LTE presents problems, however, because meeting the existing LTE requirements would cause an MTC device to be more costly and to consume more power than it would ordinarily require to satisfy its own quality of service requirements. As mentioned above, an MTC device typically requires only a low data rate for signaling a small amount of data. One example of an MTC device category is sensory equipment. An important requirement of such devices is that they should have low cost as well as low power consumption. Examples of cellular system parameters that typically drive cost and power consumption are the system bandwidth as well as response time. Using LTE as defined according to current standardization releases requires that a device support a system bandwidth that is up to 20 MHz. Supporting such a large bandwidth would increase the cost for LTE MTC devices, and such support would essentially be unnecessary from the MTC device's point of view because only a small system bandwidth (e.g., up to some few MHz) is required to support the MTC device's relatively low data rate.
Furthermore, LTE has short response time requirements, in terms of a short amount of time for issuing a Hybrid Automatic Repeat Request (HARQ) response, as well as a short time interval between the control signaling (indicating that data information is forthcoming) and the actual transmission of the data information. (In LTE systems, the PDCCH points out data in the PDSCH that is included in the same subframe as the PDCCH). Satisfying these time requirements imposes high requirements on the processing speed (which drives power) and/or the need for parallel processing (increasing baseband chip area and thereby the cost). MTC devices supporting low data rates and with low power requirements optimally should use long response times (e.g., a longer time for decoding of control information and data) in order to reduce the required clocking speed or parallel processing requirements.
The points raised above show why it is beneficial to restrict MTC devices to operate at system bandwidths that are lower than 20 MHz. But it would be too restrictive to require that all cellular networks limit themselves to using only small bandwidths if they are support power and cost efficient MTC devices.
Presently, there is an incompatibility between MTC devices supporting only a low bandwidth and/or having insufficient decoding performance (e.g., requirements of a longer delay between the PDCCH and the possible data on the PDSCH) which prevents such devices from being able to connect to an LTE system as it is presently defined by the Third Generation Partnership Project (3GPP) standard. While such MTC devices would be able to perform a cell search and receive a Master Information Block (MIB) on just a 1.4 MHz bandwidth, camping on a conventional LTE cell would still not be possible because being able to receive the further broadcast information (e.g., a System Information Block—“SIB”) that is required for the MTC device to be able to, for example, perform a random access via the Random Access Channel (RACH) requires that the MTC device be capable of supporting the full LTE bandwidth and also that the MTC device be able to decode the PDCCH and the PDSCH without any additional delay restrictions compared to the current standard.
It is therefore desirable to have methods and apparatuses that enable an MTC device to retain its relatively low performance characteristics (e.g., in terms of size of bandwidth supported and/or processing power) and yet be capable of connecting to a modern-day cellular communications system, such as but not limited to an LTE system, that ordinarily imposes higher performance requirements on connecting devices.